1. Field of the Invention
This invention relates generally to fuel cell systems, and more particularly, to control of vapors and gases involved in the operation of the fuel cell system.
2. Background Information
Fuel cells are devices in which an electrochemical reaction involving a fuel molecule is used to generate electricity. A variety of compounds may be suited for use as a fuel depending upon the specific nature of the cell. Organic compounds, such as methanol or natural gas, are attractive fuel choices due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Many currently developed fuel cells are reformer-based systems. However, because fuel processing is complex and generally requires components which occupy significant volume, reformer based systems are presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger scale applications. In many direct oxidation fuel cells, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system, or DMFC system. In a DMFC system, methanol or a mixture comprised of methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water.
Typical DMFC systems include a fuel source, fluid and effluent management sub-systems, and air management sub-systems, in addition to the direct methanol fuel cell itself (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”), which are all typically disposed within the housing.
The electricity generating reactions and the current collection in a direct oxidation fuel cell system take place within and on the MEA. In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (originating from fuel and water molecules involved in the anodic reaction) migrate through the catalyzed membrane electrolyte, which is impermeable to the electrons. The electrons travel through an external circuit, which includes the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell and water product at the cathode of the fuel cell.
A typical MEA includes a centrally disposed protonically-conductive, electronically non-conductive membrane (“PCM”, sometimes also referred to herein as “the catalyzed membrane”). One example of a commercially available PCM is Nafion® a registered trademark of E.I. Dupont de Nemours and Company, a cation exchange membrane based on polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the fuel mixture across the catalyzed anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a wet-proofed diffusion layer is used to allow a sufficient supply of oxygen by minimizing or eliminating the build-up of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assists in the collection and conduction of electric current from the catalyzed PCM.
Direct oxidation fuel cell systems for portable electronic devices should be as small as possible at the power output required. The power output is governed by the rate of the reactions that occur at the anode and the cathode of the fuel cell. More specifically, the anode process in direct methanol fuel cells based on acidic electrolytes, including polyperflourosulfonic acid and similar polymer electrolytes, involves a reaction of one molecule of methanol with one molecule of water. In this process, the oxygen atom in the water molecule is electrochemically activated to complete the oxidation of methanol to a final CO2 product in a six-electron process, according to the following chemical equationCH3OH+H2O═CO2+6H++6e−  (1)
A passive fuel cell system that uses high concentration fuel without the need for external water recirculation loops has been described in commonly-assigned U.S. patent application Ser. No. 10/413,983 filed on Apr. 15, 2003 by Ren et al. for a DIRECT OXIDATION FUEL CELL OPERATING WITH DIRECT FEED OF CONCENTRATED FUEL UNDER PASSIVE WATER MANAGEMENT, which is incorporated herein by reference. That application describes a passive direct oxidation fuel cell system that uses a passive mass transport barrier element disposed between the fuel source and the anode aspect of the catalyzed membrane electrolyte. In one embodiment of that invention, the passive mass transport barrier is described as a methanol vapor delivery film.
Another method and apparatus for delivering a vaporous fuel to a direct oxidation fuel cell was described in commonly-assigned U.S. patent application Ser. No. 10/688,433 filed on Oct. 17, 2003 by Becerra et al. for a FUEL SUBSTANCE AND ASSOCIATED CARTRIDGE FOR FUEL CELL, which is incorporated herein by reference, which describes a unique fuel substance to which a thickening agent is added to form a gel fuel. When the gel fuel is placed in a fuel refill, a highly concentrated vaporous fuel substance is delivered to a fuel cell or array of fuel cells and the associated fuel cell system. In such applications, one goal is to deliver sufficient fuel for operation to each fuel cell, and may be accomplished by feeding the fuel perpendicular to the major surface of the MEA (known as “face feeding”). Another benefit of face feeding is to maximize the even distribution of the fuel to the active anode aspect of the catalyzed membrane.
However, the rate at which fuel is delivered using face feeding fuel delivery systems is difficult to control using methods known in the art. In addition to providing an even distribution of fuel, the correct amount of fuel delivered is also important to control because the efficiency of a direct methanol fuel cell is dependent in part upon the amount of methanol present at the anode catalyst. If more methanol is present than is needed for electricity generation, the excess will not be used for electricity generation, but instead passes through the catalyzed membrane. When excess methanol crosses over the catalyzed membrane, it reacts with oxygen in the presence of the catalyst present on the cathode side, generating heat and water. This reaction is normally not desirable as it leads to the waste of fuel. In addition, excess water may result in cathode flooding, which inhibits the introduction of oxygen to the cathode aspect of the fuel cell, thus limiting the performance of the fuel cell system. Furthermore, excess heat can diminish performance of the fuel cell and fuel cell system in both the short and long term. It is further desirable to be able to control the amount of fuel delivered to the fuel cell in response to operating parameters of the system, including but not limited to the current that is demanded from the fuel cell, and ambient environmental conditions. Accordingly, improving control of the flux of methanol that is delivered to the fuel cell system is desirable.
In addition, it is difficult to stop the flow of fuel in present face feed fuel delivery systems, making it difficult to shut the fuel cell and fuel cell system down when necessary or desirable. This may be of import, for example, when a fuel cell is used as a component in a hybrid power source and the battery is fully charged, then it would be advantageous to be able to substantially completely stop the fuel feed. Thus, fuel delivery is interrupted to conserve fuel. It may be further desirable to stop the flow of fuel to the anode aspect of the MEA in response to certain environmental conditions. Some of these disadvantages may also occur in face fed systems that use a liquid fuel feed.
One manner in which fuel delivery can be controlled was described in commonlyowned U.S. patent application Ser. No. 10/413,986 by Hirsch et al. for a VAPOR FEED FUEL CELL SYSTEM WITH CONTROLLABLE FUEL DELIVERY filed on Apr. 15, 2003, which is incorporated herein by reference. An adjustable fuel delivery regulation assembly is described therein that controls the fuel delivery to the anode aspect of the catalyzed membrane using an adjustable structure that, in one embodiment, includes two correspondingly perforated components that slide with respect to one another, so that when the perforations are lined up apertures are created for fuel flow. When the perforations are completely askew, there are no openings through which fuel can flow. When aligned, fuel may be delivered through the aligned perforations at the maximum rate allowed by the area of the overlapping apertures. It is also possible to orient the perforations in a manner that allows a partial overlap, where the fuel is delivered at a desired rate that corresponds to the degree of overlap of said perforations. The structures are mechanically actuated, and the parts move relative to one another, in some embodiments, externally to the fuel cell system.
However, it may be desirable to provide a mechanism to control face feeding of a direct oxidation fuel cell where the actuation of the shutter does not include externally moving parts, and consumes less volume in the fuel cell, and is less complex mechanically. Mechanical actuators, such as those presently used increase the complexity of the system, and if motors are used to adjust the degree to which a valve is open, they require more power than is desirable. Further, the displacement of the moving parts, including but not limited to a shutter, requires additional space within the fuel cell system. As noted herein, fuel cell systems, particularly direct oxidation fuel cell systems, are particularly suited for small handheld electronic devices in which power requirements and form factors are both critical. Thus, it would be an advantage to have a suitable shutter, which does not require that large portions of the respective components move past each other along a major surface. There are also engineering challenges faced with respect to providing a substantially complete seal between moving parts.
It has otherwise been known to provide piezoelectric or other types of mechanical valves to regulate the flow of fuel into a fuel cell system, however, there are certain undesirable complexities associated with such valve types as well as with the shutters described herein.
Further, there remains a need for regulating the rate at which a vaporous fuel is fed in a vaporous fuel feed in a fuel cell system that is fed using a face feeding system. Accordingly, there remains a need for an apparatus for controlling the amount of fuel that is delivered to the anode aspect of the catalyzed membrane in a direct oxidation fuel cell system that is not mechanically driven and which has fewer moving parts and lower volume than presently available solutions. It is thus an object of the invention to provide a fuel cell system that includes such a mechanism for controlling the rate of delivery of fuel to the fuel cell.